1. Field of the Invention
This invention relates to an electrochemical process for synthesizing organic carbonates by electrolyzing a liquid medium containing a non-fluoride halide-containing electrolyte and a paraffinic monohydric or 1,2-dihydric alcohol under a carbon monoxide atmosphere.
2. Brief Description of the Prior Art
Organic carbonates, such as dimethyl carbonate, ethyl carbonate, ethylene carbonate and propylene carbonate, are a useful class of solvents and reagents. They find use in many industrial applications such as solvents for polymers and resins in processing operations, and in the synthesis of pharmaceuticals, rubber chemicals, textile finishing agents and polycarbonate resins.
Conventional methods for preparation of organic carbonates usually employ the reaction of phosgene and an alcohol at elevated temperature, as described in the Encyclopedia of Chemical Technology, Volume 4, page 391, by Kirk-Othmer (Wiley, New York, 1964).
Other known methods for producing carbonates, including the following, employ a catalyst salt for participating in a redox reaction with carbon monoxide.
U.S. Pat. No. 3,114,762 (1963) describes a process for producing carbonates by reacting carbon monoxide with a monohydric alcohol in the presence of a metal salt such as palladium bromide.
U.S. Pat. No. 3,846,468 (1974) describes a process for producing carbonates by reacting alcohol with carbon monoxide in the presence of an organometallic cuprous chloride complex.
The reference, Kondo et al., Bull. Chem. Soc. Japan, Vol. 48 (1), pp. 108-111, (1975), describes a process for producing organic carbonates by reacting alkoxides with carbon monoxide and oxygen in the presence of a selenium catalyst.
However, the above processes either require the use of large quantities of toxic phosgene or expensive metal catalyst salts, which after use are either discarded or require regeneration by a separate oxidation process for recycle.
What is desired and what the prior art does not provide is a convenient, economical process for producing organic carbonates without resort to the use of large quantities of phosgene or the use of expensive metal catalyst salts.
Electrochemical oxidation reactions have some distinct advantages over "normal" solution oxidation reactions in which a homogenous or heterogenous catalyst is used. The anode in an electrochemical reaction serves as an electron acceptor for negatively charged species in solution, thus promoting the oxidation reaction and obviating the need for oxidation catalyst salts. Furthermore, reduced species in solution can be conveniently regenerated at the anode for further participation in the oxidation reaction.
Electrochemical oxidations involving an alcohol and carbon monoxide are known.
The electrolytic carbonylation of arylated alpha olefins to produce alpha, beta-unsaturated esters, using carbon monoxide, is described in Bull. Chem. Soc. Japan, Volume 38, page 21-22 (1965). The process involves electrolyzing an alcoholic solution of an arylated alpha olefin saturated with carbon monoxide, using sodium methoxide as an electrolyte.
Anodic oxidations of methanol and ethanol are described in J. Electroanal. Chem. Vol. 31, pp. 265-267 (1971), using different electrolytes such as sodium perchlorate, tetrabutylammonium fluoride, and sodium methoxide. The products of the oxidations were found to be ethers and acetals of the corresponding starting alcohols.
The anodic oxidation of anhydrous methanol is described in J. Electrochem. Soc., Vol. 123, pp. 818-823 (1976). Anodic oxidation was carried out using sodium methoxide as the electrolyte. Under anhydrous conditions, formaldehyde was the major product, and with added water to the system, formate ion was produced.
The reference, J. Electrochem. Soc., Vol. 124, pp. 1177-1184 (1977), describes the anodic oxidation of methanol and ethanol in the presence of sodium iodide as electrolyte. The electrolysis of methanol produced primarily methyl formate and the electrolysis of ethanol produced primarily ethyl formate, along with ethyl methyl ether, methyl iodide and a trace of acetaldehyde.
However, none of the aforementioned references describe or suggest the possibility of forming organic carbonates by an electrochemical process.
We have unexpectedly found that by passing a direct electric current through a liquid medium containing a non-fluoride halide-containing electrolyte and a paraffinic monohydric or 1,2-dihydric alcohol, under a carbon monoxide atmosphere, organic carbonates are formed. The halide ion of the electrolyte, preferably being bromide ion, is essential for formation of the carbonates, and is usually used in an amount of about 0.01 to 10 weight percent, based on the amount of said alcohol used. The electrolysis is usually conducted in the temperature range from about 0.degree. C. to 100.degree. C., and under a carbon monoxide atmosphere at a pressure of about 1 to 350 atmospheres. The halide ion is believed to be continuously regenerated in the process, and thus the need for large amounts of toxic phosgene or expensive metal catalysts, as used in the prior art, is obviated.